Originally, it started on the A21 in Bromley and headed north along what is now the A2212. That road gained its number in the mid-1930s and so the present southern terminus of the B212 is on the A20 at Lee Green.
The road goes north along Lee Road, forming the boundary between the London Boroughs of Lewisham and Greenwich. In Blackheath it joins the B220 at a mini-roundabout then descends to go over the railway line by Blackheath station.
There is a large gyratory on the far side which takes the road to the southern edge of the actual heath. The B212 continues along Prince of Wales Road to cross the A2 at traffic lights. It then becomes urban once more and terminates at a gyratory with the B210 and B211.